Former White House ethics attorney Walter Shaub sounded alarms Friday in response to growing calls on the political right, and rumblings from within the White House, for Special Counsel Robert Mueller to be ousted amid suggestions that bias is tainting the investigation into alleged Russian interference in last year’s election and possible Trump campaign collusion with that effort.

“The coordinated effort by President Trump and his surrogates to discredit the Mueller investigation raises serious alarms,” said Shaub, now the senior ethics director at the Campaign Legal Center, in a statement. “Rather than making themselves complicit in this assault on the rule of law, Members of Congress should send a clear message to the President that firing Mueller is a red line he must not cross.”

His comments came after Trump’s attorney called for a second special counsel to examine links between the Justice Department and Fusion GPS, a private firm that carried out its own research regarding Trump’s ties with Russia.